The present invention relates to a video signal processor or a processing scheme that utilizes digital techniques to obtain a distortion corrected digital signal that is ultimately converted into an idealized analog signal.
It has been established that analog signals, such as video and audio waveform signals, can be represented digitally, by a series of numbers, provided that two constraints are met. The first constraint is that the number of bits in the digital word, representing the waveform, must be sufficient to accurately describe the signal amplitude at any point in time. This constraint is met by choosing analog to digital (A to D) and digital to analog (D to A) converters with sufficient resolution. The second constraint is that the sampling rate must be equal to or greater than twice the highest frequency component of the analog signal. This second constraint is met by choosing a high enough sampling frequency.
If both these constraints are met, the signal theoretically can be reconstructed essentially unaltered. Reconstruction of a digitized signal is accomplished by first using D to A converter and then filtering the analog output of that converter.
The problem with the current state of the art is that the filter following the D to A converter must limit signals having frequencies above the one-half sampling frequency to minimal amplitudes while passing all other signal frequencies without alteration. It is difficult to construct, manufacture, design and implement such filters since the required rapid roll off of high frequency signals, i.e., the filter response, introduces significant phase distortions in the components of the analog signal. These phase distortions result in serious over shoot and ringing on transient waveforms. the traditional solution to this problem is to incorporate expensive precision phase correction filters as filter stages in the output filter following the D to A converter. These phase correction filter stages must be hand tuned to guarantee good performance and the accurate reproduction of the analog video signal.
The present invention relates to a video signal generator or a video signal processor. A video signal generator is a device which produces an idealized, standardized analog video signal at its output. This analog video signal is input into a video monitor in order to determine whether the monitor and associated equipment is correctly adjusted to reproduce the appropriate levels of luminance, chrominance, that is brightness and color of the video signal, among other things. The most critical use of the video signal generator is in the broadcast industry. As used herein, the term "standardized video signal" refers to an analog video signal representative of the SMPTE color bar signal or other idealized or test analog video signal recognized in the industry or utilized in the adjustment of video monitors by the industry or by an individual user. The claims are meant to cover such analog signals. SMPTE color bars refers to Socity of Motion Picture and Television Engineers modification (document ECR 1-1978) of EIA-RS189A. The term "video signal" refers to a base band signal and hence is not solely limited to signals utilized exclusively for visual representations. The claims appended hereto are meant to cover signals similar to video signals.
The present invention also relates to a video signal processor. Prior art devices currently process video signals in real time. Some of these prior art devices are frame synchronizer delay generators or time base correctors. These digital signal processors adjust the frame or other timing characteristics of a line, frame or field of the video signal. As will be explained later, the video signal processor in accordance with the present invention can be utilized in conjunction with digitally configured frame synchronizer devices and the time base correction devices.
In both video signal generators and the video signal processors, the video signal is, at some point, represented by a digital signal. Ultimately, this digital signal must be converted into an analog signal such that the video monitor can reproduce the analog video signal. The digital video signal is, as stated earlier, generated by an output filter following the D to A converter. This output filter, in the prior art devices is typically an 8, 10 or 12 pole filter. Generally, these filters are initially designed to limit the higher frequency signal to prescribed amplitudes (such that the higher frequencies are significantly attenuated but the lower frequency signals are passed) and the last few stages (2 or 4 poles of the filter) compensate for phase changes or distortions that occur due to the frequency limiting earlier stages of the filter. These phase correction poles or filter stages are designed as all pass frequencies. It is these filter stages, the all pass networks in the output filter, that must be tuned by hand. Therefore, the prior art output filters are expensive to design, manufacture and furthermore, expensive to incorporate within electronic equipment.